So-called inboard-outboard drives for boats have been in use for many years. Examples of such drives are shown in U.S. Pat. Nos. 1,798,596, 2,415,813, 2,755,766, 2,977,923, 3,088,296, 3,382,838, 3,888,203, 3,893,407, 3,933,116 and 3,951,096. These inboard-ouboard drives are used in propelling boats generally having large inboard engines. A first type of drive, the type shown in all of the above patents, except for the type of drive shown in U.S. Pat. Nos. 2,415,183 and 3,933,116, has a drive shaft extending through the transom of a boat and connected by gears to a generally vertically extending shaft which in turn is connected by gears to the propeller shaft. A second type of drive, the type shown in U.S. Pat. Nos. 2,415,183 and 3,933,116, has a drive shaft which extends through the transom of a boat and connects directly to a propeller shaft without using a vertical shaft as in the other patents. For example, the propeller supporting member of the first type of drive can be rotatably lifted when the boat is in shallow water or for inspection and maintenance of the propeller and its shaft. An advantage of a drive of the first type is that the trim of the boat may be adjusted by rotating the propeller supporting member about a horizontal axis. In addition to the tilting feature, the drive is rotatable about a generally vertical axis to steer the boat. A common arrangement provides a universal joint about which the propeller supporting member of the drive of the first type can be both tilted and steered, as for example the arrangement shown in the aforementioned U.S. Pat. No. 3,088,296.
Although conventional inboard-outboard drives of the first type mentioned above have some advantages and have been commercially successful to some extent, they also have disadvantages. For example, such drives are relatively heavy, expensive to manufacture and maintain, and are inefficient in transferring power from the engine to the propeller. A power loss of as much as 17% can occur because of transfer losses through the gears and couplings as compared to power losses with a direct drive. Moreover, since the propeller supporting member of such drives generally extends a considerable distance below the surface of the water, such drives have appreciable drag.
To overcome some disadvantages of conventional inboard-outboard drives of the first type, the second type of drive mentioned above has been developed. Such a drive eliminates the generally vertical shaft of the first type and couples the drive shaft directly to the propeller shaft. These direct drives, however, involve mechanisms which are too complex to be of commercial success. For example, the drive of U.S. Pat. No. 3,933,116 uses a surface piercing propeller keyed to a propeller shaft that is moved by the articulation of a gimbal assembly about a horizontal axis for steering and about a vertical axis to trim the boat. The gimbal assembly gives only limited control of the propeller shaft because the rings of the gimbal assembly are constrained to move about respective, fixed, mutually perpendicular axes. The design of this drive also requires the drive shaft to be disposed at an appreciable distance above the bottom of the transom of a boat. Thus, the propeller shaft must assume a vertically tilted position thereby pushing the bow of the boat downwardly at relatively high speeds.
The drive of U.S. Pat. No. 2,415,813 provides a ball swivel unit connected to the transom of a boat. Control of the propeller shaft of this drive is severely limited because of the location of the swivel unit and the lack of steering devices attached to the propeller shaft support.